Method for producing a multi metal oxide catalyst, method for producing unsaturated aldehydes and/or carboxylic acids and band calcination device

ABSTRACT

A catalyst suitable for the gas-phase oxidation of organic compounds to α,β-unsaturated aldehydes and/or carboxylic acids and having an active phase comprising a multimetal oxide material is prepared by a process in which a particulate catalyst precursor which contains oxides and/or compounds of the elements other than oxygen which constitute the multimetal oxide material, which compounds can be converted into oxides, is prepared and said catalyst precursor is converted by calcination into a catalytically active form, wherein a stream of the particulate catalyst precursor is passed at substantially constant speed through at least one calcination zone at constant temperature for calcination.

The present invention relates to a process for the preparation of acatalyst comprising an active phase of a multimetal oxide material, aprocess for the preparation of α,β-monoethylenically unsaturatedaldehydes and/or carboxylic acids using the catalyst and a beltcalcination apparatus which is designed especially for the preparationof the catalyst.

While acrylic acid and acrylates were prepared predominantly by theReppe process even toward the end of the 60 s, the preparation bygas-phase oxidation of propene predominates today. The oxidation ofpropene to acrylic acid can be carried out in one stage or two stages.Catalysts used for the heterogeneously catalyzed reaction are as a rulemultimetal oxide materials which generally contain heavy metalmolybdates as main component and compounds of various elements aspromoters. The oxidation of propene takes place in a first step to giveacrolein and in a second step to give acrylic acid. Since the twooxidation steps may differ in their kinetics, uniform process conditionsand a single catalyst do not as a rule lead to optimum selectivity.Recently, two-stage processes with optimum adaptation of catalyst andprocess variables have therefore preferably been developed. In general,propene is oxidized to acrolein in the presence of molecular oxygen inthe first stage in an exothermic reaction in a fixed-bed tubularreactor. The reaction products are passed directly into the secondreactor and are further oxidized to acrylic acid. The reaction gasesobtained in the second stage can be condensed and the acrylic acid canbe isolated therefrom by extraction and/or distillation.

The oxidation of propene to acrolein and/or acrylic acid is highlyexothermic. The tubes of the fixed-bed tubular reactor which are filledwith the heterogeneous catalyst are therefore surrounded by a coolingmedium, as a rule a salt melt, such as a eutectic mixture of KNO₃ andNaNO₂. The heat of reaction is released through the wall of thecatalyst-filled tubes to the salt bath. In spite of the cooling by thecooling medium, a uniform temperature is not established over the lengthand/or the cross section of the catalyst-filled tubes. Overheatedregions, i.e. hot spots, form. Thus, the propene concentration of thereaction mixture in the vicinity of the entrance of the catalyst-filledtubes is high while the reaction mixture in the vicinity of the exit hasa lower propene concentration, promoting the formation of overheatedregions in the vicinity of the entrance. Since the reaction rategenerally increases with increasing temperature, an inhomogeneoustemperature distribution leads to additional conversion in the warmerregions, the exothermic heat evolution associated therewith leading togreater disequilibrium of the temperature.

The formation of overheated regions in the catalyst-filled tubes isdisadvantageous for various reasons. Excessive oxidation beyond thedesired oxidation state occurs in the overheated regions. Furthermore,the excessive thermal stress influences the catalyst performance andcatalyst life. The formation of overheated region can finally lead to anuncontrolled course of the reaction, which can result in an explosiverunaway reaction. The excessive formation of overheated regions isdisadvantageous in particular in the case of molybdenum-based catalysts,since molybdenum compounds tend to sublime.

For standardizing the temperature profile in the catalyst-filled tubes,it has been proposed to use catalysts of different activities in thetubes, the catalyst activities increasing axially in the direction offlow of the reaction mixture. Catalysts of different activity can beobtained, for example, by varying the chemical composition, cf. U.S.Pat. No. 5,276,178 and DE 30 06 894. On the other hand, the catalystactivity can also be varied by varying the calcination temperatureduring the catalyst preparation, cf. WO 98/24746. U.S. Pat. No.5,198,581 recommends using catalysts of different volume ratios, thevolume ratio decreasing axially in the direction of flow of the reactionmixture. These proposals are disadvantageous in that the preparation ofa plurality of catalysts of different activity and the structuredfilling of the catalyst into the reaction tubes is complicated andtime-consuming.

U.S. Pat. No. 4,438,217 recommends the use of specific catalystgeometries before reducing the problems of the formation of overheatedregions. EP 0 807 465 describes a catalyst having a defined proportionof active phase, particle size of the catalyst and calcinationtemperature.

JP 08086571-A describes a treatment apparatus which comprises a beltconveyor having a gas-permeable belt, which revolves within a hollowcylindrical housing, and an apparatus by means of which a gas can bepassed to the conveying part of the belt inside the path of therevolving belt.

It is an object of the present invention to provide a catalyst having anactive phase of a multimetal oxide material and a process for itspreparation which, when used for the preparation ofα,β-monoethylenically unsaturated aldehydes and/or carboxylic acids,permits substantial optimization of the equilibrium of activity,selectivity and catalyst life.

It was found that the problems described above in connection with theformation of the hot spots in the case of catalysts prepared by knownprocesses are superimposed and exacerbated by fluctuations in theactivity of the individual catalyst particles over a given population ofcatalyst particles which are contained in an industrial fixed-bedtubular reactor. The heterogeneous catalysts used and comprising anactive phase of a multimetal oxide material have, depending on theirchemical composition and preparation conditions, a temperature range inwhich activity, selectivity and catalyst life are optimally balanced. Ifparticles having greatly differing activities are present in apopulation of the catalyst particles in a reactor, part of the catalystis inevitably operated at a temperature which is more or less far awayfrom the optimum temperature range of the respective catalyst particles.This is associated with declines in the yield, selectivity and catalystlife.

With the present invention, it was found that the use of a population ofcatalyst particles having a substantially homogeneous activity, i.e.,having a very narrow activity distribution, permits optimization of thepreparation of α,β-monoethylenically unsaturated aldehydes and/orcarboxylic acids with regard to yield, selectivity and catalyst life.Usually, an industrial reactor for the preparation ofα,β-monoethylenically unsaturated aldehydes and/or carboxylic acidscontains several tons of catalyst. The prior art provides no indicationas to how a very narrow activity distribution can be achieved over sucha large amount of catalyst. The apparatuses usually used for thecalcination, such as muffle furnaces or tray furnaces, either permitonly the preparation of small amounts of catalyst or, if they have asufficiently large capacity, have an insufficient temperature constancy.Since, as shown in the examples and comparative examples below, evensmall differences in the calcination temperature can lead to greatlyvarying activity, insufficient temperature constancy during thecalcination leads to catalyst batches having greatly fluctuatingactivity. The mixing of a large amount with catalyst of fluctuatingactivity to standardize the activity has substantial disadvantages. Onthe one hand, the catalyst is exposed, during the mixing process, tomechanical stress which leads to abrasion in the case of coatedcatalysts and to breakages in the case of extrudates and catalystpellets. On the other hand, only standardization at a low level isachieved by mixing since activity differences between the individualcatalyst particles also remain on mixing.

We have found that this object is achieved by a process for the simpleand economical preparation of large amounts of a catalyst comprising anactive phase of a multimetal oxide material having a narrow activitydistribution.

The present invention relates to a process for the preparation of acatalyst suitable for the gas phase oxidation of organic compounds toα,β-unsaturated aldehydes and/or carboxylic acids and having an activephase of a multimetal oxide material, in which a particulate catalystprecursor which contains oxides and/or compounds of the elements otherthan oxygen which constitute the multimetal oxide material, whichcompounds can be converted into oxides, is prepared and said catalystprecursor is converted by calcination into a catalytically active form,wherein a stream of the particulate catalyst precursor is passed atsubstantially constant speed through at least one calcination zone forcalcination, the maximum variation of the temperature as a function oftime and the maximum local temperature difference in the calcinationzone each being ≦5° C.

Catalysts prepared according to the invention may be present both asunsupported catalysts and as supported catalysts. In the case of theunsupported catalysts, the catalyst substantially comprises themultimetal oxide material. The corresponding catalyst precursorsubstantially comprises a thorough mixture of oxides and/or compounds ofthe elements other than oxygen which constitute the multimetal oxidematerial, which compounds can be converted into oxides. The mixture iscompacted to the desired catalyst geometry, for example by pelleting orextrusion, if required with the use of conventional assistants, such aslubricants and/or molding assistants, such as graphite or stearic acid,or reinforcing materials, such as microfibers of glass, asbestos,silicon carbide or calcium titanate. The unsupported catalysts may haveany desired geometry, such as cylindrical, spherical, etc.; preferredcatalyst geometries are hollow cylinders, for example having an externaldiameter and a length of from 2 to 10 mm and a wall thickness of from 1to 3 mm.

In order to obtain catalyst precursors for supported catalysts preparedaccording to the invention, supports are expediently coated with agenerally thorough powder mixture of oxides and/or compounds of theelements other than oxygen which constitute multimetal oxide material,which compounds can be converted into oxides. Expediently, the powdermaterial to be applied can be moistened for coating the supports and,after application, can be dried, for example by means of hot air. Thecoat thickness of the powder material applied to the support isexpediently chosen to be in the range from 50 to 500 μm, preferably from100 to 350 μm. The support materials used may be conventional porous ornonporous aluminas, silica, thorium dioxide, zirconium dioxide, siliconcarbide or silicates, such as magnesium silicate or aluminum silicate.The supports may have a regular or irregular shape, regularly shapedsupports having preferably substantial surface roughness, e.g. spheresor hollow cylinders, being preferred.

The multimetal oxide material may be present in amorphous and/orcrystalline form. It may contain crystallites of a defined crystalstructure type or mixtures of such crystallites. The multimetal oxidematerial may have a substantially uniform composition of the mixture ormay contain regions of a dispersed phase whose chemical compositiondiffers from that of its local environment. This is the case, forexample, with key phases or promoter phases which are dispersed in ahost phase. The regions of differing chemical composition preferablyhave a maximum diameter of from 1 to 25 μm, in particular from 1 to 20μm, and particularly preferably from 5 to 15 μm, the maximum diameterbeing regarded as the longest connecting line between two points presenton the surface of the particle which passes through the center ofgravity of a particle. As a rule, the dispersed phase is preparedseparately beforehand in finely divided form and mixed with oxidesand/or compounds of the elements other than oxygen which constitute thehost phase of the multimetal oxide material, which compounds can bedecomposed into oxides.

Particularly suitable compounds, which as a rule can be converted intooxides by heating in the presence or absence of oxygen, are halides,nitrates, formates, oxalates, citrates, acetates, carbonates, aminecomplex salts, ammonium salts and/or hydroxides. Compounds such asammonium hydroxide, ammonium carbonate, ammonium nitrate, ammoniumformate, ammonium acetate, ammonium oxalate or acetic acid, whichdecompose and/or can be decomposed on calcination substantiallycompletely to give compounds escaping in gaseous form, can beadditionally incorporated as pore formers. The oxides and/or compoundsof the elements other than oxygen which constitute the multimetal oxidematerial, which compounds can be converted into oxides, are preferablythoroughly mixed. The thorough mixing can be effected in dry or in wetform. If it is effected in dry form, the starting compounds areexpediently used in the form of finely divided powders. Preferably,however, the thorough mixing is effected in wet form. Usually, thestarting compounds are mixed with one another in the form of an aqueoussolution and/or suspension. The aqueous material obtained is then dried,the drying process preferably being carried out by spray-drying of theaqueous mixture.

Catalysts comprising an active phase of a multimetal oxide material forthe preparation of α,β-monoethylenically unsaturated aldehydes and/orcarboxylic acids by gas-phase oxidation of an alkane, alkanol, alkeneand/or alkenal of 3 to 6 carbon atoms are known per se and aredescribed, for example, in EP 0 000 835, EP 0 575 897, DE 198 55 913,U.S. Pat. No. 5,276,178, DE 30 06 894, U.S. Pat. No. 5,198,581, WO98/24746, U.S. Pat. No. 4,438,217 and EP 0 807 465.

These catalysts are suitable in particular for the preparation ofα,β-monoethylenically unsaturated aldehydes and/or carboxylic acids in ahigh-load procedure.

The multimetal oxide materials preferably contain at least one firstmetal selected from molybdenum and tungsten and at least one secondmetal selected from bismuth, tellurium, antimony, tin, copper, iron,cobalt and/or nickel.

Particularly preferred multimetal oxide materials have the formula I orII

[X¹ _(a)X² _(b)O_(x)]_(p)[X³ _(c)X⁴ _(d)X⁵ _(e)X⁶ _(f)X⁷ _(g)X²_(h)O_(y]q)  (I)

Mo₁₂Bi_(i)X⁸ _(k)Fe_(l)X⁹ _(m)X¹⁰ _(n)O_(z)  (II)

where

-   X¹ is bismuth, tellurium, antimony, tin and/or copper, preferably    bismuth,-   X² is molybdenum and/or tungsten,-   X³ is an alkali metal, thallium and/or samarium, preferably    potassium,-   X⁴ is an alkaline earth metal, nickel, cobalt, copper, manganese,    zinc, tin, cadmium and/or mercury, preferably nickel and/or cobalt,-   X⁵ is iron, chromium, cerium and/or vanadium, preferably iron,-   X⁶ is phosphorus, arsenic, boron and/or antimony,-   X⁷ is a rare earth metal, titanium, zirconium, niobium, tantalum,    rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium,    indium, silicon, germanium, lead, thorium and/or uranium, preferably    silicon, aluminum, titanium and/or zirconium,-   a is from 0.01 to 8,-   b is from 0.1 to 30,-   c is from 0 to 4,-   d is from 0 to 20,-   e is from 0 to 20,-   f is from 0 to 6,-   g is from 0 to 15,-   h is from 8 to 16,-   x and y are numbers which are determined by the valency and    frequency of the elements other than oxygen in I,-   p and q are numbers whose ratio p/q is from 0.1 to 10,-   X⁸ is cobalt and/or nickel, preferably cobalt,-   X⁹ is silicon and/or aluminum, preferably silicon,-   X¹⁰ is an alkali metal, preferably potassium, sodium, cesium and/or    rubidium, in particular potassium,-   i is from 0.1 to 2,-   k is from 2 to 10,-   l is from 0.5 to 10,-   m is from 0 to 10,-   n is from 0 to 0.5,-   z is a number which is determined by the valency and frequency of    the elements other than oxygen in II.

The novel process is particularly suitable for the preparation ofcatalysts comprising an active phase of a multimetal oxide material ofthe formula II, since these materials have a particularly pronounceddependence of the activity on the calcination temperature.

Multimetal oxide materials of the formula I are known per se from EP 0000 835 and EP 0 575 897 and multimetal oxide materials of the formulaII are known per se from DE 198 55 913; these publications are herebyincorporated by reference in their entirety.

Multimetal oxide materials of the formula I contain three-dimensional,delimited regions of the chemical composition X¹ _(a)X² _(b)O_(x), whosechemical composition differs from that of their local environment. Themaximum diameter of the delimited regions is preferably from 1 to 25 μm,in particular from 1 to 20 μm, and particularly preferably from 5 to 15μm. Catalyst precursors for catalysts comprising an active phase of amultimetal oxide material of the formula I are obtained in a suitablemanner by first preforming a calcined oxide X¹ _(a)X² _(b)O_(x) bymixing, for example, water-soluble salts of X¹ with oxo acids or theirammonium salts of X² in aqueous solution, drying the solution andcalcining the dried material, if necessary comminuting the oxideobtained and separating off particles of a desired particle class. Theseparately preformed oxide is then mixed with oxides and/or compounds ofX³, X⁴, X⁵, X⁶, X⁷ and X² which can be converted into oxides, in desiredstoichiometric ratios, and, if the mixing is effected in the wet state,the mixture is dried.

Catalyst precursors for catalysts comprising an active phase of amultimetal oxide material of the formula II are obtained in a suitablemanner by preparing a first aqueous solution of compounds of theelements bismuth, iron and X⁸, preparing a second aqueous solution ofcompounds of the elements molybdenum and X¹⁰, mixing the first andsecond aqueous solutions, the first or the second aqueous solution or amixture thereof being mixed with a solution or suspension of a compoundof the element X⁹, and drying the resulting mixture or precipitatedproduct, preferably by spray-drying.

For the final calcination, a stream of the particulate catalystprecursor, preferably a mass flow which is substantially constant as afunction of time, is passed through at least one calcination zone atsubstantially constant speed. The term calcination is intended here alsoto include thermal treatment steps, such as final drying and/ordecomposition, which are upstream of the actual calcination.Expediently, the stream of the particulate catalyst precursor is passedin succession through at least two, for example from two to ten,calcination zones, which as a rule are thermostated at differenttemperatures. In this way, it is possible to realize differenttemperature profiles through which the stream of the particulatecatalyst precursor passes. For example, the successive calcination zonescan be thermostated, for example, at temperatures increasing stepwise.The individual calcination zones may have different spatial dimensions,so that different residence times in the individual calcination zonesresult at constant speed of the stream of the particulate catalystprecursor.

In at least one calcination zone, the catalyst precursor is heated to atemperature of in general from 400 to 600° C., preferably from 450 to530° C. When the novel process is carried out using eight calcinationzones, for example, the following temperature profile is suitable:

1: 100-200° C.; 2: 150-250° C.; 3: 200-300° C.; 4: 250-350° C.; 5:350-400° C.; 6: 400-550° C.; 7: 400-550° C.; 8: 400-550° C.

With the use of three zones, the following temperature profile issuitable:

1: 100-400° C.; 2: 250-550° C.; 3: 400-550° C.

With the use of four zones, the following temperature profile issuitable:

1: 100-250° C.; 2: 200-350° C.; 3: 350-550° C.; 4: 400-550° C.

With the use of six zones, the following temperature profile issuitable:

1: 100-200° C.; 2: 150-300° C.; 3: 200-350° C.; 4: 300-400° C.; 5:400-500° C.; 6: 400-600° C.

With the use of 12 zones, the following temperature profile is suitable:

1: 100-200° C.; 2: 150-250° C.; 3: 200-300° C.; 4: 250-350° C.; 5:350-400° C.; 6: 400-500° C.; 7: 400-550° C.; 8: 400-550° C.; 9: 400-600°C.; 10: 400-600° C.; 11: 400-700° C.; 12: 400-700° C.

In order to carry out the novel process successfully, it is essentialfor the temperature in the calcination zone (in the calcination zones)to be substantially constant locally and as a function of time. Thetemperature constancy can be checked by means of temperature measuringapparatuses, for example thermocouples. Preferably at least 4, inparticular at least 6, particularly preferably at least 10,thermocouples are provided per calcination zone and are arranged as faras possible equidistant over the calcination zone. Preferably, the meanvalue of the hourly mean values of the temperature measured at theindividual thermocouples of a calcination zone differs by not more than5° C., in particular not more than 3° C., particularly preferably notmore than 2° C., from the respective setpoint value. The record of thetemperatures measured at the individual thermocouples and thecalculation of the mean values are expediently carried out automaticallyby an appropriately programmed computer. This advantageously alsoregulates or controls the heating of the calcination zones. Thethermocouples are advantageously regularly calibrated in order to ensurethat the maximum deviation of the measured temperature from the actualtemperature is preferably less than 0.5° C.

The maximum variation of the temperature in the calcination zone (in thecalcination zones) as a function of time is ≦5° C., in particular ≦3°C., particularly preferably ≦2° C. The variation of the temperature as afunction of time is regarded as the standard deviation of thetemperature measured at an individual thermocouple of a calcination zoneover one hour. The maximum local temperature difference, i.e. themaximum temperature difference within the calcination zone (in thecalcination zones) is ≦5° C., in particular ≦3° C., particularlypreferably ≦2° C. The local temperature difference is regarded as thestandard deviation of the hourly mean values of the temperaturesmeasured at the thermocouples of a calcination zone.

The atmosphere in the calcination zone may consist of inert gas, e.g.nitrogen or argon, a mixture of inert gas and oxygen, e.g. air, reducinggases, such as hydrocarbons, e.g. methane, aldehydes, e.g. acrolein orammonia, or of a mixture of oxygen and reducing gases. In thecalcination under reducing conditions, however, it should be ensuredthat the metallic constituents are not reduced to the element.Expediently, an oxygen-containing gas, in particular air, is thereforepresent in the calcination zone. In order to achieve the uniformtemperature in the calcination zone and/or to transport away anydecomposition gases formed, it is preferable to pass a stream of a gasover the catalyst precursor, perpendicularly to the direction of advanceof the catalyst precursor in the calcination zone. Suitable gases arethe abovementioned ones, in particular air. Particularly for thepreparation of catalysts comprising a molybdenum- and bismuth-containingmultimetal oxide material, it may be advantageous to carry out thecalcination in an atmosphere which contains 1% by volume or more ofnitric oxide and 0.5% by volume or more of oxygen, as described inEP-A-558 028.

For carrying out the novel process, it is expedient to use a beltcalcination apparatus comprising at least one heatable chamber and aconveyor belt passing through the chamber and intended for holding theparticulate catalyst precursor. The conveyor belt is as a rule acontinuous belt which can be advanced horizontally through the heatablechamber and forms at its one end an ascending reverse path and at itsother end a descending reverse path. It is often advantageous to providea housing within which the conveyor belt revolves, i.e. which surroundsthe chamber(s) and the ascending and descending reverse paths. Means forfeeding the particulate catalyst precursor onto the conveyor belt areexpediently provided in the vicinity of the ascending reverse path andmeans for removing the calcined catalyst from the conveyor belt areexpediently provided at the descending reverse path. The means forfeeding consists, for example, of a shaft filled with the particulatecatalyst precursor, a bed of the catalyst precursor having asubstantially constant layer thickness being withdrawn under the shaftby the movement of the conveyor belt. The shaft expediently has ascraping apparatus, for example an adjustable weir. At the descendingreverse path, the calcined catalyst is ejected from the conveyor beltand expediently collected by suitable means. If, as in a preferredembodiment, the gas is passed over the catalyst precursor in thecalcination zone, the conveyor belt is gas-permeable and consists, forexample, of a perforated belt or woven fabric or knitted fabric of metalwire or another heat-resistant material. The speed of the conveyor beltis as a rule from about 0.1 to 5 cm/min. In order to pass theparticulate catalyst precursor in succession through a plurality ofcalcination zones of different temperatures, a belt calcinationapparatus which comprises at least two, for example from two to ten,heatable chambers which can be regulated to different temperatures isexpediently used.

The heating of the chamber(s) can be effected in various ways, forexample by electrical heating by means of resistance elements, forexample installed in the chamber wall. However, the heating ispreferably effected indirectly, i.e. by means of heat sources, such asburners, which are arranged outside the chamber(s), suitably below thechamber(s). The burners are operated with a combustible gas, such asnatural gas.

In the chamber (the chambers) of the belt calcination apparatus, thecalcination zone is defined by the width of the support area of theconveyor belt and the maximum bed height of the particulate catalystprecursor on the conveyor belt. The thermocouples provided formonitoring the temperature constancy in the calcination zone arepreferably distributed uniformly over the support area of the conveyorbelt and the bed height. In order to achieve very high local temperatureconstancy in the calcination zone, the atmosphere in the heatablechambers is preferably circulated. A belt calcination apparatusspecially designed for carrying out the novel process therefore has, inthe chamber, means for producing a gas circulation on the basis offorced convection. Such a belt calcination apparatus as such is noveland is a further subject of the present invention.

The present invention therefore relates, in a further aspect, to a beltcalcination apparatus comprising at least one heatable chamber and agas-permeable conveyor belt passing through the chamber and intended forholding a particulate material, wherein means for producing a gascirculation on the basis of forced convection are provided in thechamber.

In a preferred embodiment, the means for producing the gas circulationcomprise a fan, which is suitably arranged above the conveyor belt inthe chamber (the chambers). In suitable embodiments, the means forproducing the gas circulation also comprise gas-guiding apparatuses forguiding the gas circulation inside the chamber, the gas-guidingapparatuses inside the chamber extending in each case along the edge ofthe conveyor belt, substantially in a plane perpendicular to the supportarea of the conveyor belt. The means for producing the gas circulationand/or the gas-guiding apparatuses are expediently formed in such a waythat the gas ascends through the gas-permeable conveyor belt and the bedof particulate catalyst precursor present thereon and descends againalong the walls of the chamber. However, a gas circulation in theopposite direction is also possible. If the belt calcination apparatushas at least two heatable chambers, these are preferably delimitedrelative to one another in such a way that substantially no gas exchangetakes place between the chambers. For removal of decomposition gases andthe like, some of the gas circulating in the chamber is preferablyremoved continuously or periodically and replaced by fresh gas. Thesupply of fresh gas is controlled in such a way that the temperatureconstancy in the chamber is not adversely affected. The volume of thegas circulated by unit time in the chamber is as a rule greater than thevolume of the gas fed into or removed from the chamber per unit time andis preferably at least five times the amount.

If desired, it is also possible to pass in succession through aplurality of the belt calcination apparatuses described above, forexample two or three thereof, in order to optimize the different heatsof reaction in individual steps, for example by varying the bed heightand/or residence time. After passing through one apparatus and beforepassing through a further apparatus, the catalyst precursor can, ifrequired, be collected and temporarily stored.

By means of the novel process, large amounts of a catalyst which issuitable for the preparation of α,β-monoethylenically unsaturatedaldehydes and/or carboxylic acids by gas-phase oxidation of organiccompounds, e.g. alkanes, alkanols, alkenes and/or alkenals of 3 to 6carbon atoms and has an active phase of a multimetal oxide material canbe obtained with a narrow activity distribution within the batchprepared. For the purposes of the present invention, the activity of thecatalyst is defined as the temperature at which a propene conversion of95% is obtained if a mixture of 5% by volume of propene, 9.5% by volumeof oxygen and 85.5% by volume of nitrogen is passed over 100 g ofcatalyst at 100 l (S.T.P.)/h (volume at standard temperature (0° C.) andpressure (1013 mbar)). An experimental setup as described further belowis suitable for the determination.

In a further aspect, the present invention relates to a batch of atleast 100 kg, in particular at least 300 kg, particularly preferably atleast 1 t (metric ton) of a catalyst which has a multimetal oxidematerial of the abovementioned formula I or II as the active phase, thestandard deviation of the activity, as defined above, of any randomsamples taken being less than 7° C., in particular less than 3.5° C.,particularly preferably less than 2.5° C.

The number of random samples is at least 5, in particular at least 10,for example at least one random sample per 100 kg production quantity.The size of the random sample is preferably about 1 kg, from which,after homogenization, 100 g are taken for the abovementioned activitydetermination. In the particularly preferred case, the mean value of theactivity can be estimated at ±1.6° C. with a reliability of 95% in thecase of the production of 1000 kg of product and taking of a randomsample from each 100 kg container and subsequent homogenization of therandom samples.

For storage or transport, the catalyst batch can suitably be filled intoplastic or sheet metal containers which have a capacity of, for example,50 l or 100 l. In many cases, it is expedient to take a sample of, forexample, 1 kg per container for the random sampling discussed above.

Catalysts prepared according to the invention are particularly suitablefor the preparation of α,β-monoethylenically unsaturated aldehydesand/or carboxylic acids by gas-phase oxidation of alkanes, alkanols,alkenes and/or alkenals of 3 to 6 carbon atoms. They are furthermoresuitable for the preparation of nitriles by ammoxidation, in particularof propene to acrylonitrile and of 2-methylpropene or tert-butanol tomethacrylonitrile. They are furthermore suitable for the oxidativedehydrogenation of organic compounds.

The catalysts prepared according to the invention are suitable inparticular for the preparation of acrolein, acrylic acid, methacroleinand methacrylic acid, the starting compounds used preferably beingpropene or 2-methylpropene, tert-butanol or methacrolein. Catalystsprepared according to the invention are particularly suitable for thepreparation of acrolein from propene. Oxygen, expediently diluted withinert gases, is used as the oxidizing agent in a manner known per se.Examples of suitable inert gases are nitrogen and steam. Suitablereaction temperatures and reaction pressures are known to a personskilled in the art, it being possible to specify the temperature of from250 to 450° C. and a pressure of from 0.5 to 4 bar (gauge pressure) asgeneral ranges.

In a further aspect, the present invention relates to a process for thepreparation of α,β-monoethylenically unsaturated aldehydes and/orcarboxylic acids, in which a gaseous stream of an alkane, alkanol,alkene and/or alkenal of 3 to 6 carbon atoms is passed, at elevatedtemperatures in the presence of molecular oxygen, through at least onereaction zone which contains a batch of a catalyst having the narrowactivity distribution defined above.

The catalysts prepared according to the invention are particularlysuitable for the high-load procedure. The loading of the catalyst withthe alkane, alkanol, alkene and/or alkenal is at least 160, inparticularly at least 165, particularly preferably at least 170, 1(S.T.P.) per 1 of catalyst per hour.

Advantageously, the gaseous stream is passed in succession through afirst and a second reaction zone, the temperature of the first reactionzone being from 300 to 330° C. and the temperature of the secondreaction zone being from 300 to 365° C. and being at least 5° C. abovethe temperature of the first reaction zone. The first reaction zoneextends as a rule to a conversion of the alkane, alkanol, alkene and/oralkenal of from 40 to 80, preferably from 50 to 70, mol %.

The attached figures and the examples which follow illustrate theinvention.

FIG. 1 shows a longitudinal section and

FIG. 2 a cross section through a novel belt calcination apparatus.

With reference to FIG. 1, the calcination apparatus shown has fourchambers (1, 2, 3, 4), through which a conveyor belt (5) runs. Theapparatus has a bunker (6) and a weir (7). Above the conveyor belt (5),fans (8) are present in each chamber. Each chamber is provided with airfeed and exhaust means (9). During the operation, the bunker (6) isfilled with the particulate catalyst precursor. As a result of themovement of the conveyor belt (5), a layer of the catalyst precursorwith constant bed height is withdrawn under the weir (7) and passed insuccession through the chambers of the calcination apparatus.

With reference to FIG. 2, each chamber is heated by burners (9). Gasdeflection plates (10) which, together with the fans (8), ensure thatthe circulated atmosphere in each chamber ascends through thegas-permeable conveyor belt (5) and descends again along the walls ofthe chamber are arranged at the edges of the conveyor belt,substantially in a plane perpendicular to the support area of theconveyor belt.

The catalysts prepared in the examples are characterized by theparameters conversion, acrolein selectivity (S_(ACR)) and acrylic acidselectivity (S_(ACA)). These parameters were determined as follows:

First, a 15 cm long bed of steatite beads having a diameter of from 2 to3 mm was introduced into a steel pipe having an internal diameter of 15mm and an external diameter of 20 mm. 100 g of the catalyst to be testedwere then introduced, an average bed height of from 60 to 70 cmresulting. The steel pipe was heated by means of a nitrogen-flushed saltbath over the total length of the bed comprising the catalyst and thesteatite beads. At a constant feed gas rate of 100 l (S.T.P.)/h (volumeat standard temperature and pressure) and with a composition of 5% byvolume of propene, 9.5% by volume of oxygen and 85.5% by volume ofnitrogen, the temperature of the salt bath was varied until the propeneconversion measured at the exit of the steel pipe was 95%. The propeneconversion is defined as follow:

${{conv}._{propene}\lbrack\%\rbrack} = {\frac{c_{{propene},\mspace{14mu} {exit}}}{c_{{propene},\mspace{14mu} {entrance}}} \times 100}$

In addition, the selectivities with respect to the desired productsacrolein and acrylic acid were measured. These are calculated asfollows:

${S_{ACR}\lbrack\%\rbrack} = {\frac{c_{{ACR},\mspace{14mu} {exit}}}{c_{{propene},\mspace{14mu} {entrance}} - c_{{propene},\mspace{14mu} {exit}}} \times 100}$${S_{ACS}\lbrack\%\rbrack} = {\frac{c_{{ACS},\mspace{14mu} {exit}}}{c_{{propene},\mspace{14mu} {entrance}} - c_{{propene},\mspace{14mu} {exit}}} \times 100}$

The selectivity with respect to the desired product is calculated as:

S _(DP)[%]=S _(ACA) +S _(ACR)

EXAMPLE 1

First, a starting material of the host phase was prepared. For thispurpose, 244 kg of ammonium heptamolybdate were dissolved in portions at60° C. in 660 l of water, and 1.12 kg of a 47.5% strength by weightpotassium hydroxide solution at 20° C. were added while stirring. Asecond solution was prepared by adding 133.8 kg of an iron nitratesolution (13.8% by weight of iron) to 288.7 kg of a cobalt nitratesolution (12.5% by weight of cobalt), the temperature being kept at600C. The second solution was added to the molybdate solution within aperiod of 30 minutes at 60° C. 15 minutes after the end of the addition,19.16 kg of silica sol (density 1.36-1.42 g/ml, from pH 8.5 to 9.5,alkali content not more than 0.5% by weight; 46.80% by weight of SiO₂)were added to the slurry obtained. Stirring was then carried out for 15minutes. The slurry obtained was then spray-dried, a powder having aloss on ignition (3 hours at 600° C.) of about 30% by weight beingobtained.

A finely divided promoter phase was then prepared. 209.3 kg of tungsticacid (72.94% by weight of tungsten) were then added in portions, whilestirring, to 775 kg of a solution of bismuth nitrate in nitric acid(free nitric acid 3-5%, density 1.22-1.27 g/ml, 11.2% by weight ofbismuth). The slurry obtained was stirred for 2 hours and was dried byspray-drying. A powder having a loss on ignition (3 hours at 600° C.) of12% by weight was obtained. This powder was converted into a paste witha small amount of water in a kneader and was converted by means of anextruder into extrudates. These were cut into 6 cm sections and thencalcined in a rotary tubular furnace at from 700 to 900° C. for 2 hours,then milled to a mean particle size of about 51m and mixed with 1% byweight of finely divided silica (tapped density 150 g/l, mean particlesize 10 μm, BET surface area 100 m²/g).

The promoter phase prepared beforehand and the starting material of thehost phase were mixed, with admixing of 1.5% by weight of graphite (min.50% by weight <24 μm; 24 μm<max. 10% by weight <48 μm; max. 5% byweight >48 μm, according to sieve analysis; BET surface area 6-13 m²/g),in a ratio so that a material having stoichiometry[Bi₂W₂O₉×2WO₃]_(0.5)[Mo₁₂Co_(5.5)Fe_(2.94)Si_(1.59)K_(0.08)O_(y)]₁ wasobtained. Cylindrical rings having an external diameter of 5 mm, aheight of 3 mm and a hole diameter of 2 mm were formed from thematerial.

The shaped rings were placed in a bed height of from 50 to 70 mm on thebelt of a belt calcination apparatus having eight chambers. The chamberseach had a fan for producing an air circulation and were thermostated at180° C., 200° C., 290° C., 390° C., 465° C., 465° C. and 435° C.,respectively. Within the chambers, the deviation of the temperature fromthe setpoint value as a function of time and location was always ≦2° C.The length of the chambers was such that the residence time in the firstfour chambers was 1.5 h in each case and that in the fifth to eighthchamber was 2 h in each case. In this way, 2.6 t of catalyst wereprepared. A 1 kg spot sample was taken from each container having acontent of 100 kg and was characterized as described above. The resultsobtained are shown in Table 1.

TABLE 1 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 315 92.7 2 317 93  3 317 92.3  4 317 92.3  5 319 92.6  6 319 93.6  7 320 94  8322 94.7  9 316 93.7 10 319 94.1 11 321 94.1 12 320 92.8 13 322 93.6 14321 94.9 15 320 92.9 16 317 93 17 317 92.6 18 320 93 19 320 92.7 20 31792.9 21 318 93.9 22 320 93.2 23 318 92.7 24 322 93.9 25 320 92.3 26 31794 Mean value 319 93.3 Standard deviation 0.62% 0.80%

COMPARATIVE EXAMPLE 1

The catalyst precursor from Example 1 was calcined in portions of 500 gin a conventional laboratory muffle furnace, in which the temperaturefluctuation as a function of time and location was ±3° C., at differentrequired temperatures for the duration of 6 hours. The catalystproperties as a function of the calcination temperature are shown inTable 2.

TABLE 2 Experiment T_(calcination) [° C.] Activity [° C.] S_(ACR+ACA)[mol %] 1 443 314 84.5 2 446 311 92.3 3 449 309 93.6 4 460 321 94.7 5467 329 96.2 6 471 330 96.7 7 473 330 96.6 8 480 377 96.3 9 480 343 96.9

Table 2 shows that, for example, a temperature difference of 7° C. inthe calcination (473° C. and 480° C.) can lead to an activity differenceof 47° C. (330° C. and 377° C.). Temperature differences of this orderof magnitude are unavoidable in the calcination of large amounts ofcatalyst by conventional processes, as with the use of tray furnaces.Experiments 8/9 show that different results are obtained even at thesame required temperature, since the actual temperature is veryinhomogeneous without forced convection, even in the case of 500 gportions.

COMPARATIVE EXAMPLE 2

300 kg of the catalyst precursor from Example 1 were distributed over 9trays arranged one on top of the other on a trolley. The trolley waspushed into a furnace and heated in an air stream. Heating was effectedto 185° C. in the course of one hour, the temperature was kept at thisvalue for 1 hour, heating was then effected to 210° C. at a rate of 2°C./min, the temperature was kept at this value for 2 hours, heating wasthen effected to 250° C. at a rate of 2° C./min, the temperature waskept at this value for 1 hour, heating was then effected to 465° C. at arate of 5° C./min and the temperature was kept at this value for 6hours. Said temperatures are the required temperatures set on thefurnace. Different temperatures deviating therefrom prevailed on theindividual trays of the trolley. From each of these lots, an averagesample of all individual trays was taken and was tested as follows.

TABLE 3 Lots Activity [° C.] S_(ACR+ACA) [mol %]  1 330 94.7  2 339 95.6 3 324 95.2  4 341 94.8  5 334 94.8  6 344 95.1  7 330 95  8 340 94.9  9345 95.4 10 332 94.6 11 332 94.2 12 346 96.1 13 350 96.2 14 332 96.3 15331 95.3 16 330 94.8 17 330 95.1 18 323 95.6 19 345 95.6 20 335 95.6 21328 94.8 22 334 95.8 23 329 95.5 24 328 96.8 25 334 95.3 26 342 96.5 27331 94.7 28 336 94.9 29 326 95 30 338 95.1 31 326 94.2 32 331 95.3 33322 94.6 34 330 95.6 35 345 95.4 36 323 96 37 333 95.7 38 337 95.7 39340 95.8 40 322 94 41 343 95.1 42 334 94.5 43 335 95.1 44 336 94.4 45345 95.2 46 338 95.6 47 343 95.3 48 344 96.4 49 345 95.8 50 348 96.3 51350 95.9 52 350 96.4 53 346 96.1 54 345 96.6 55 338 95.3 56 344 96.1 57342 96.1 58 341 96.4 59 348 96 60 344 94.9 61 348 96.2 62 339 94.4 63328 93.8 64 340 94.9 65 332 95.5 66 332 95 67 334 95.1 68 344 95.4 69334 95.1 70 333 94.8 71 337 94.6 72 337 95.5 73 344 94.7 74 328 94.5 75325 94.3 76 343 95.5 77 329 95.5 78 340 96.3 79 336 96 80 342 95.3 81331 94.9 82 341 95.1 83 335 94.7 84 341 94.7 85 331 94.8 86 329 95 87324 94.4 88 333 95.6 89 333 94.1 90 334 94.2 Mean value 336.3 95.3Standard deviation 2.168% 0.703%

EXAMPLE 2

First, a starting material of the host phase was prepared. For thispurpose, 213 kg of ammonium heptamolybdate were dissolved in portions at60° C. in 600 l of water, and 0.97 kg of a 46.8% strength by weightpotassium hydroxide solution at 20° C. were added while stirring. Asecond solution was prepared by adding 80.2 kg of an iron nitratesolution (14.2% by weight of iron) to 262.9 kg of a cobalt nitratesolution (12.4% by weight of cobalt), the temperature being kept at600C. The second solution was added to the molybdate solution within aperiod of 30 minutes at 60° C.15 minutes after the end of the addition,19.16 kg of silica sol (density 1.36-1.42 g/ml, from pH 8.5 to 9.5,alkali content not more than 0.5% by weight; 46.80% by weight of SiO₂)were added to the slurry obtained. Stirring was then carried out for 15minutes. The slurry obtained was then spray-dried, a powder having aloss on ignition (3 hours at 600° C.) of about 30% by weight beingobtained.

The preparation of the finely divided promoter phase was carried out asin Example 1.

The promoter phase prepared beforehand and the starting material of thehost phase were mixed, with admixture of 1.5% by weight of graphite, ina ratio so that a material having a stoichiometry[Bi₂W₂O₉×2WO₃]_(0.5)[Mo₁₂Co_(5.5)Fe_(2.0)Si_(1.59)K_(0.08)O_(y]) ₁ wasobtained. The cylindrical rings having an external diameter of 5 mm, aheight of 2 mm and a hole diameter of 2 mm were formed from thematerial. The shaped rings were calcined as described in Example 1. Thecharacterization of the catalyst obtained was also carried out asdescribed in Example 1. The results obtained are shown in Table 4.

TABLE 4 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 320 93.7 2 316 92.6  3 322 93.1  4 322 92.9  5 321 92.3  6 319 94.0  7 320 92.5 8 317 92.1  9 320 92.5 10 318 92.8 11 320 92.7 12 318 92.2 13 321 93.914 318 93.5 15 322 93.9 16 324 93.8 17 317 92.5 18 318 92.9 19 322 93.920 321 92.7 Mean value 320 93.0 Standard deviation 0.67% 0.69%

COMPARATIVE EXAMPLE 3

The catalyst precursor from Example 2 was calcined in portions of 500 gin a conventional laboratory muffle furnace, in which the temperaturefluctuation as a function of time and location was ±3° C., at differentrequired temperatures for the duration of 6 hours. The catalystproperties as a function of the calcination temperature are shown inTable 5.

TABLE 5 Experiment T_(calcination) [° C.] Activity [° C.] S_(ACR+ACA)[mol %] 1 440 316 83 2 445 312 92.1 3 450 310 93.4 4 460 321 94.2 5 467327 96.4 6 471 330 96.7 7 475 333 96.8 8 477 340 97.5 9 480 370 96.5

EXAMPLE 3

213 kg of ammonium heptamolybdate were dissolved in portions at 60° C.in 600 l of water, and 0.97 kg of a 46.8% strength by weight potassiumhydroxide solution at 20° C. was added while stirring. A second solutionwas prepared by adding 116.25 kg of an iron nitrate solution (14.2% byweight of iron) to 333.7 kg of a cobalt nitrate solution (12.4% byweight of cobalt), the temperature being kept at 30° C. and stirringbeing carried out for a further 30 minutes up to the end of theaddition. At 60° C., 112.3 kg of a bismuth nitrate solution (11.2% ofbismuth) are metered into the iron/cobalt solution. The second solutionwas added to the molybdate solution within a period of 30 minutes at 60°C. 15 minutes after the end of the addition, 19.16 kg of silica sol(46.80% by weight of SiO₂) were added to the slurry obtained. Stirringwas then carried out for 15 minutes. The slurry obtained was thenspray-dried, a powder having a loss on ignition (3 hours at 600° C.) ofabout 30% by weight being obtained.

The composition of the active material isMo₁₂Co₇Fe_(2.94)Bi_(0.6)Si_(1.59)K_(0.08)O_(z.)

After the spray-drying, the starting material was mixed with 1.5% byweight of graphite, compacted and formed into cylindrical rings havingan external diameter of 5 mm, a height of 3 mm and a hole diameter of 2mm. The shaped rings were calcined in a belt calcination apparatushaving eight chambers. The chambers were thermostated at 160° C., 200°C., 230° C., 270° C., 380° C., 430° C., 500° C. and 500° C.,respectively. The residence time was 2 hours each in the first to fourthchambers and 5 hours in the fifth to eighth chambers. Thecharacterization of the catalyst obtained was carried out as inExample 1. The results obtained are shown in Table 6.

TABLE 6 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 319 95.6 2 318 95.7  3 317 95.7  4 317 95.3  5 319 95.5  6 320 95.4  7 319 95.7 8 319 95.4  9 319 95.4 10 318 95.4 11 319 95.4 12 321 95.8 13 320 95.814 320 95.7 15 323 96.0 16 318 95.8 17 321 96.1 Mean value 319 95.6Standard deviation 0.48% 0.24%

COMPARATIVE EXAMPLE 4

The catalyst precursor from Example 3 was calcined in portions of 500 gin a conventional laboratory muffle furnace, in which the temperaturefluctuation as a function of time and location was about ±3° C., atdifferent required temperatures for the duration of 6 hours. Thecatalyst properties as a function of the calcination temperature areshown in Table 7.

TABLE 7 Experiment T_(calcination) [° C.] Activity [° C.] S_(ACR+ACA)[mol %] 1 461 309 94.3 2 462 307 95.2 3 466 313 94.3 4 467 317 95.1 5469 313 95.4 6 471 316 96 7 479 324 96.7 8 485 326 96.5 9 491 324 96.910 493 331 97.3 11 500 347 97.4

EXAMPLE 4

Example 1 was repeated, but a belt calcination apparatus having 12 zoneswas used. The temperature and residence time in the first eight zoneswere as stated in Example 1. Zones 9 to 12 were all thermostated at 500°C., and the residence time was 2 hours in each case. The atmosphere inchambers 9 to 12 substantially comprised nitrogen. In this way, 3.5 t ofcatalyst were prepared. A 1 kg spot sample was taken from each containerhaving a content of 100 kg and was characterized as described above. Theresults obtained are shown in Table 8.

TABLE 8 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 315 94.7 2 316 93  3 317 94.3  4 317 94.3  5 319 94.6  6 318 93.8  7 319 94.6  8317 93.7  9 314 93.4 10 318 95 11 320 95.1 12 320 95.2 13 318 94.8 14317 94.3 15 314 94 16 318 94.8 17 318 94.8 18 318 94.7 19 317 94.3 20314 94 21 319 94.5 22 320 95 23 320 94.8 24 318 94.7 25 317 94.6 26 31694.7 27 319 95 28 317 94.5 29 319 95 30 314 94.4 31 316 94.6 32 317 94.733 317 94.8 34 318 94.7 35 319 95 Mean value 317.4 94.5 Standarddeviation 0.552% 0.515%

EXAMPLE 5

191.5 kg of MoO₃, 45.6 kg of CoO, 27.2 kg of Fe₂O₃, 10.4 kg of SiO₂, and0.4 kg of K₂O were thoroughly mixed in succession in a mixer; thepromoter phase prepared in Example 1 was then added in a ratio so that amaterial having a stoichiometry[(Bi₂W₂O₉×2WO₃)_(0.5)/Mo₁₂Co_(5.5)Fe_(3.0)Si_(1.6)K_(0.08)O_(y)] wasobtained, said material being formed to cylindrical rings having anexternal diameter of 5 mm, a height of 2 mm and a hole diameter of 2 mm.The shaped rings were calcined in a belt calcination apparatus which hadthree chambers which were thermostated at 270° C., 465° C. and 465° C.,respectively. The residence time per chamber was 3 hours. In this way,2.5 t of catalyst were prepared. A 1 kg spot sample was taken from eachcontainer having a content of 100 kg and was characterized as describedabove. The results obtained are shown in Table 9.

TABLE 9 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 317 93.7 2 318 94  3 317 94.3  4 319 95  5 317 94  6 320 94.5  7 320 94  8 32194.8  9 317 93 10 316 93 11 317 93.4 12 319 93.8 13 315 93 14 317 93.615 318 93.8 16 319 93.8 17 319 94 18 318 93.5 19 317 93.4 20 315 93.8 21319 93 22 316 93.6 23 320 93.8 24 318 93 25 317 92.8 Mean value 317.893.7 Standard deviation 0.495% 0.609%

EXAMPLE 6

Example 3 was repeated, but the amounts of the components were chosen sothat the chemical composition Mo₁₂Co₇Fe₃Bi_(1.0)Si_(1.6)K_(0.08)O_(z)was obtained. The calcination was carried out in a belt calcinationapparatus having eight zones which were thermostated at 160° C., 210°C., 240° C., 290° C., 380° C., 515° C., 515° C. and 400° C.,respectively. A 1 kg spot sample was taken from each container having acontent of 100 kg and was characterized as described above. The resultsobtained are shown in Table 10.

TABLE 10 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 317 94.9 2 318 95.8  3 319 94.7  4 317 95.1  5 319 95.2  6 320 94.8  7 321 96  8318 94.8  9 319 94.8 10 317 95.7 11 316 95 12 318 95.6 13 318 95 14 31995.7 15 317 94.9 16 319 95.3 17 318 95 18 317 94.7 19 318 95 20 319 95.621 319 95.7 22 320 95.7 23 320 95.8 24 318 95 25 317 94.8 Mean value318.3 95.2 Standard deviation 0.382% 0.440%

EXAMPLE 7

114.4 kg of MoO₃, 34.5 kg of CoO, 15.8 kg of Fe₂O₃, 6.3 kg of SiO₂, 9.2kg of Bi₂O₃ and 0.2 kg of K₂O were homogeneously mixed in succession ina mixer. The mixture was formed to cylindrical rings having an externaldiameter of 5 mm, a height of 3 mm and a hole diameter of 2 mm.

The shaped rings were calcined in a belt calcination apparatus havingthree chambers which were thermostated at 350° C., 510° C. and 510° C.,respectively. The residence time per chamber was 2.5 hours. In this way,2.5 t of catalyst were prepared. A 1 kg spot sample was taken from eachcontainer having a content of 100 kg and was characterized as describedabove. The results obtained are shown Table 11.

TABLE 11 Consecutive No. Activity [° C.] S_(ACR+ACA) [mol %]  1 316 94.9 2 318 95.8  3 317 94.7  4 319 95.1  5 316 95.2  6 320 94.8  7 318 96  8321 94.8  9 318 94.8 10 315 95.7 11 317 95 12 319 95.6 13 315 95 14 31895.7 15 318 94.9 16 319 95.3 17 319 95 18 320 94.7 19 317 95 20 315 95.621 319 95.7 22 315 95.7 23 320 95.8 24 318 95 25 316 94.8 Mean value317.7 95.2 Standard deviation 0.557% 0.440%

1. A process for the preparation of a catalyst suitable for the gasphase oxidation of organic compounds to α,β-unsaturated aldehydes and/orcarboxylic acids and having an active phase of a multimetal oxidematerial, in which a particulate catalyst precursor which containsoxides and/or compounds of the elements other than oxygen whichconstitute the multimetal oxide material, which compounds can beconverted into oxides, is prepared and said catalyst precursor isconverted by calcination into a catalytically active form, wherein astream of the particulate catalyst precursor is passed at substantiallyconstant speed through at least one calcination zone for calcination,the maximum variation of the temperature as a function of time and themaximum local temperature difference in the calcination zone each being<5° c.
 2. A process as claimed in claim 1, wherein the multimetal oxidematerial contains at least one first metal selected from molybdenum andtungsten and at least one second metal selected from bismuth, tellurium,antimony, tin, copper, iron, cobalt and/or nickel.
 3. A process asclaimed in claim 2, wherein the multimetal oxide material has theformula I or II,[X¹ _(a)X² _(b)O_(x]) _(p)[X³ _(c)X⁴ _(d)X⁵ _(e)X⁶ _(f)X⁷ _(g)X²_(h)O_(y)]_(q)  (I)Mo₁₂Bi_(i)X⁸ _(k)Fe_(l)X⁹ _(m)X¹⁰ _(n)O_(z)  (II) where X¹ is bismuth,tellurium, antimony, tin and/or copper, X² is molybdenum and/ortungsten, X³ is an alkali metal, thallium and/or samarium, X⁴ is analkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin,cadmium and/or mercury, X⁵ is iron, chromium, cerium and/or vanadium, X⁶is phosphorus, arsenic, boron and/or antimony, X⁷ is a rare earth metal,titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium,silver, gold, aluminum, gallium, indium, silicon, germanium, lead,thorium and/or uranium, a is from 0.01 to 8, b is from 0.1 to 30, c isfrom 0 to 4, d is from 0 to 20, e is from 0 to 20, f is from 0 to 6, gis from 0 to 15, h is from 8 to 16, x and y are numbers which aredetermined by the valency and frequency of the elements other thanoxygen in I, p and q are numbers whose ratio p/q is from 0.1 to 10, X⁸is cobalt and/or nickel, X⁹ is silicon and/or aluminum, X¹⁰ is an alkalimetal, i is from 0.1 to 2, k is from 2 to 10, l. is from 0.5 to 10, m isfrom 0 to 10, n is from 0 to 0.5, z is a number which is determined bythe valency and frequency of the elements other than oxygen in II.
 4. Aprocess as claimed in any of the preceding claims, in which a gas streamis passed through the catalyst precursor stream perpendicularly to thedirection of advance of the catalyst precursor in the calcination zone.5. A process as claimed in any of the preceding claims, in which thecatalyst precursor is passed through at least two calcination zoneswhich are thermostated at different temperatures.
 6. A process asclaimed in any of the preceding claims, in which the maximum variationof the temperature in the calcination zone as a function of time is ≦3°C.
 7. A process as claimed in claim 6, in which the maximum variation ofthe temperature in the calcination zone as a function of time is ≦2° C.8. A process as claimed in any of the preceding claims, in which themaximum local temperature difference in the calcination zone is ≦3° C.9. A process as claimed in claim 9, in which the maximum localtemperature difference in the calcination zone is ≦2° C.
 10. A batch ofa catalyst, which has a multimetal oxide material of the formula I or IIstated in claim 3 as the active phase, of least 100 kg, the standarddeviation of the activity of any random samples taken from the batch,expressed as the temperature at which a propene conversion of 95% isobtained if a mixture of 5% by volume of propene, 9.5% by volume ofoxygen and 85.5% by volume of nitrogen passed over 100 g of catalyst at100 l (S.T.P.)/h, being less than 7° C.
 11. A process for thepreparation of α,β-monoethylenically unsaturated aldehydes and/orcarboxylic acids, in which a gaseous stream of an alkane, alkanol,alkene and/or alkenal of 3 to 6 carbon atoms is passed, at elevatedtemperatures in the presence of molecular oxygen, through at least onereaction zone which contains at least one batch of a catalyst as claimedin claim
 10. 12. A process as claimed in claim 11, in which the loadingof the catalyst with the alkane, alkanol, alkene and/or alkenal is atleast 160 l (S.T.P.) per l of catalyst per hour.
 13. A process asclaimed in claim 11 or 12, in which the alkene is propene, and acroleinis obtained.
 14. A process as claimed in any of claims 11 to 13, inwhich the gaseous stream is passed in succession through a first and asecond reaction zone, the temperature of the first reaction zone beingfrom 300 to 330° C. and the temperature of the second reaction zonebeing from 300 to 365° C. and being at least 5° C. above the temperatureof the first reaction zone, and the first reaction zone extending to aconversion of the alkane, alkanol, alkene and/or alkenal of from 40 to80 mol %.
 15. A belt calcination apparatus comprising at least oneheatable chamber and a gas-permeable conveyor belt passing through thechamber and intended for holding the particulate material, wherein meansfor producing a gas circulation based on forced convection are providedin the chamber.
 16. A belt calcination apparatus as claimed in claim 15,wherein the means comprise a fan.
 17. A belt calcination apparatus asclaimed in claim 15 or 16, wherein the means comprise gas-guidingapparatuses for guiding the gas circulation inside the chamber, and thegas-guiding apparatuses inside the chamber each extend along the edge ofthe conveyor belt, substantially in a plane perpendicular to the supportarea of the conveyor belt.
 18. A belt calcination apparatus as claimedin any of claims 15 to 17, wherein at least two heatable chambers whichcan be regulated to different temperatures are provided.
 19. A beltcalcination apparatus as claimed in any of claims 15 to 18, wherein thevolume of the gas circulating in the chamber per unit time is greaterthan the volume of the gas fed into or removed from the chamber per unittime.